HTTP headers wrongly treated as case-sensitive by client

[AlyoshaVasilieva]

EspHttpResponse defines headers as BTreeMap<String, String>:

esp-idf-svc/src/http/client.rs

Lines 314 to 317 in a39c119

	pub struct EspHttpResponse<'a> {
	client: &'a mut EspHttpClient,
	headers: BTreeMap<String, String>,
	}

If a server sends Content-Type: something, then resp.content_type() returns None because embedded_svc asks for content-type and EspHttpResponse does a case-sensitive search:

esp-idf-svc/src/http/client.rs

Lines 337 to 339 in a39c119

	self.headers
	.get(name.as_ref())
	.map(|s| Cow::Borrowed(s.as_str()))

I think the fix is replacing the String key with something like Uncased

[ivmarkov]

Uncased seems to cover our use case perfectly. Are you up for a PR?

[AlyoshaVasilieva]

I'll try to work on it within the next day or two

[sehraf]

What is the advantage of uncased over something simple like to_lowercase()?

[AlyoshaVasilieva]

Uncased doesn't require any additional allocations or modification of strings